307 research outputs found
The dark matter halo of NGC 1399 - CDM or MOND?
Central galaxies in galaxy clusters may be key discriminants in the
competition between the cold dark matter (CDM) paradigm and modified Newtonian
dynamics (MOND). We investigate the dark halo of NGC 1399, the central galaxy
of the Fornax cluster, out to a galactocentric distance of 80 kpc. The data
base consists of 656 radial velocities of globular clusters obtained with
MXU/VLT and GMOS/Gemini, which is the largest sample so far for any galaxy. We
performed a Jeans analysis for a non-rotating isotropic model. An NFW halo with
the parameters r_s = 50 kpc and rho_s = 0.0065 M_sun/pc^3 provides a good
description of our data, fitting well to the X-ray mass. More massive halos are
also permitted that agree with the mass of the Fornax cluster as derived from
galaxy velocities. We compare this halo with the expected MOND models under
isotropy and find that additional dark matter on the order of the stellar mass
is needed to get agreement. A fully radial infinite globular cluster system
would be needed to change this conclusion. Regarding CDM, we cannot draw firm
conclusions. To really constrain a cluster wide halo, more data covering a
larger radius are necessary. The MOND result appears as a small-scale variant
of the finding that MOND in galaxy clusters still needs dark matter.Comment: 4 pages, 2 figures, accepted for publication as a Letter in A&
Remarks on the properties of elliptical galaxies in modified Newtonian dynamics
Two incorrect arguments against MOND in elliptical galaxies could be that the
equivalent circular velocity curves tend to become flat at much larger
accelerations than in spiral galaxies, and that the Newtonian dark matter halos
are more concentrated than in spirals. Here, we compare published scaling
relations for the dark halos of elliptical galaxies to the scaling relations
expected for MONDian phantom halos. We represent the baryonic content of
galaxies by spherical profiles, and their corresponding MONDian phantom halos
by logarithmic halos. We then derive the surface densities, central densities,
and phase space densities and compare them with published scaling relations. We
conclude that it is possible to get flat circular velocity curves at high
acceleration in MOND, and that this happens for baryonic distributions
described by Jaffe profiles in the region where the circular velocity curve is
flat. Moreover, the scaling relations of dark halos of ellipticals are
remarkably similar to the scaling relations of phantom halos of MOND.Comment: Accepted for publication in A and
Tomographic filtering of highâresolution mantle circulation models: Can seismic heterogeneity be explained by temperature alone?
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95052/1/ggge1509.pd
Global S-wave tomography using receiver pairs: An alternative to get rid of earthquake mislocation
International audienceGlobal seismic tomography suffers from uncertainties in earthquake parameters routinely published in seismic catalogues. In particular, errors in earthquake location and origin-time may lead to strong biases in measured body wave delay-times and significantly pollute tomographic models. Common ways of dealing with this issue are to incorporate source parameters as additional unknowns into the linear tomographic equations, or to seek combinations of data to minimize the influence of source mislocations. We propose an alternative, physically-based method to desensitize direct S-wave delay-times to errors in earthquake location and origin-time. Our approach takes advantage of the fact that mislocation delay-time biases depend to first order on the earthquake-receiver azimuth, and to second order on the epicentral distance. Therefore, for every earthquake, we compute S-wave differential delay-times between optimized receiver pairs, such that a large part of their mislocation delay-time biases cancels out (for example origin-time fully subtracts out), while the difference of their sensitivity kernels remains sensitive to the model parameters of interest. Considering realistic, randomly distributed source mislocation vectors, as well as various levels of data noise and different synthetic Earths, we demonstrate that mislocation-related model errors are highly reduced when inverting for such differential delay-times, compared to absolute ones. The reduction is particularly rewarding for imaging the upper-mantle and transition zone. We conclude that using optimized receiver pairs is a suitable, low cost alternative to get rid of errors on earthquake location and origin-time for teleseismic direct S-wave traveltimes. Moreover, it can partly remove unilateral rupture propagation effects in cross-correlation delay-times, since they are similar to mislocation effects
Chloe Song / music by Reginald DeKoven; words by Harry B. Smith
Cover: photo of Miss Anna Held; Music Supplement of the New York Journal and American, Sunday, Jan. 5, 1902; Publisher: Eduard Schuberth and Co. (New York)https://egrove.olemiss.edu/sharris_b/1017/thumbnail.jp
The dark matter halo of NGC 1399 - CDM or MOND?
Context: Central galaxies in galaxy clusters may be key discriminants in the competition between the cold dark matter (CDM) paradigm and modified Newtonian dynamics (MOND).
Aims: We investigate the dark halo of NGC 1399, the central galaxy of the Fornax cluster, out to a galactocentric distance of 80 kpc.
Methods: The data base consists of 656 radial velocities of globular clusters obtained with MXU/VLT and GMOS/Gemini, which is the largest sample so far for any galaxy. We performed a Jeans analysis for a non-rotating isotropic model.
Results: An NFW halo with the parameters rs = 50 kpc and gs = 0.0065 Mâ/pc3 provides a good description of our data, fitting well to the X-ray mass. More massive halos are also permitted that agree with the mass of the Fornax cluster as derived from galaxy velocities. We compare this halo with the expected MOND models under isotropy and find that additional dark matter on the order of the stellar mass is needed to get agreement. A fully radial infinite globular cluster system would be needed to change this conclusion.
Conclusions: Regarding CDM, we cannot draw firm conclusions. To really constrain a cluster wide halo, more data covering a larger radius are necessary. The MOND result appears as a small-scale variant of the finding that MOND in galaxy clusters still needs dark matter.Facultad de Ciencias AstronĂłmicas y GeofĂsicasInstituto de AstrofĂsica de La Plat
The globular cluster system of NGC 1316. II - The extraordinary object SH2
SH2 has been described as an isolated HII-region, located about 6.5 arcmin
south of the nucleus of NGC 1316 (Fornax A), a merger remnant in the the
outskirts of the Fornax cluster of galaxies. We give a first, preliminary
description of the stellar content and environment of this remarkable object.
We used photometric data in the Washington system and HST photometry from the
Hubble Legacy Archive for a morphological description and preliminary aperture
photometry. Low-resolution spectroscopy provides radial velocities of the
brightest star cluster in SH2 and a nearby intermediate-age cluster. SH2 is not
a normal HII-region, ionized by very young stars. It contains a multitude of
star clusters with ages of approximately 0.1 Gyr. A ring-like morphology is
striking. SH2 seems to be connected to an intermediate-age massive globular
cluster with a similar radial velocity, which itself is the main object of a
group of fainter clusters. Metallicity estimates from emission lines remain
ambiguous. The present data do not yet allow firm conclusions about the nature
or origin of SH2. It might be a dwarf galaxy that has experienced a burst of
extremely clustered star formation. We may witness how globular clusters are
donated to a parent galaxy.Comment: 5 pages, to appear in A&A, format slightly different from the printed
versio
Large-scale study of the NGC 1399 globular cluster system in Fornax
We present a Washington C and Kron-Cousins R photometric study of the globular cluster system of NGC 1399, the central galaxy of the Fornax cluster. A large areal coverage of 1 square degree around NGC 1399 is achieved with three adjoining fields of the MOSAIC II Imager at the CTIO 4-m telescope, Working on such a large field, we can perform the first indicative determination of the total size of the NGC 1399 globular cluster system. The estimated angular extent, measured from the NGC 1399 centre and up to a limiting radius where the areal density of blue globular clusters falls to 30 per cent of the background level, is 45 ± 5 arcmin, which corresponds to 220-275 kpc at the Fornax distance. The bimodal colour distribution of this globular cluster system, as well as the different radial distribution of blue and red clusters, up to these large distances from the parent galaxy, are confirmed. The azimuthal globular cluster distribution exhibits asymmetries that might be understood in terms of tidal stripping of globulars from NGC 1387, a nearby galaxy. The good agreement between the areal density profile of blue clusters and a projected dark-matter NFW density profile is emphasized.Facultad de Ciencias AstronĂłmicas y GeofĂsica
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